Convert CONFIG_ENV_IS_IN_SPI_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_SPI_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/mach-mvebu/Kconfig b/arch/arm/mach-mvebu/Kconfig
index 01d700b..1b12b33 100644
--- a/arch/arm/mach-mvebu/Kconfig
+++ b/arch/arm/mach-mvebu/Kconfig
@@ -32,6 +32,7 @@
 config ARMADA_XP
 	bool
 	select ARMADA_32BIT
+	imply ENV_IS_IN_SPI_FLASH
 
 # ARMv8 SoCs...
 config ARMADA_3700